It is well-known that friction materials which are intended for use in heavy duty brake or clutch operations in motor vehicles must withstand severe service conditions. In use they are subjected to vigorous treatment by repeated and prolonged braking or clutching applications which develop high temperatures, usually above 500.degree. F., in the friction materials, these temperatures sometimes exceeding 1000.degree. F. on the friction surface of the material and progressively decreasing inwardly of such surface. These high temperatures, especially when occurring during high speed stops or following repeated applications of the brakes or clutches, tend to decompose the organic materials which act as a binder in the friction material. Such decomposition results in the formation of gaseous or liquid products which in some cases cause loss of braking or clutching efficiency. Moreover, the weakened, decomposed lining surfaces may wear faster with resultant premature failure of these surfaces to perform their intended function.
In general, a friction material contains a matrix or binder such as a thermosetting resin, a high-melting fibrous material, and various additives which contribute friction and/or wear properties and bulk. In the past the fibrous material has invariably been chrysotile asbestos. Asbestos fiber imparts strength to the friction material and, at the same time, has a beneficial effect on friction properties. Moreover, in the past, it has been relatively cheap.
However, asbestos deposits throughout the world are rapidly being depleted, with the result that its cost is increasing. Moreover, the continued use of asbestos has recently come under a cloud because of the realization that it presents a serious cancer hazard for those who work with it. There has been an increasing concern about such hazard with the result that the expense of health precautions now required for the safe handling of asbestos has become a significant burden. Accordingly, it has become desirable to eliminate asbestos from frictional material formulations and to find a replacement.
U.S. Pat. No. 3,967,037 (Marzocchi et al.) deals with brake linings wherein the friction material comprises glass fibers bonded with a cured organic binder, a heat-conducting particulate metal and a binder-modifying agent. An example of such a composition is shown at column 3, lines 48-57 containing glass fibers, a phenolic resin, an "elastomer", cuprous oxide and antimony sulfide.
U.K. Pat. No. 2,003,088 deals with disk brake pads and a process for their production. The brake pad comprises a friction layer and backplate layer bonded thereto. On page 2 of this patent there appears a list of several different ingredients from which the friction layer can be prepared, including phenol-formaldehyde resins, acrylonitrile rubber, copper oxide, antimony sulfide, molybdenum sulfide, etc. The preferred fibrous ingredient, however, appears to be asbestos.